The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic, electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
In order to meet the ever increasing demand for improved data rate and data capacity, magnetic heads (read and write) have been made ever smaller. Reducing the size of such sensor increases the amount of data that can be stored on a disk. For example, reducing the track width of the sensor and the write element increases the number of data tracks that can be recorded onto a disk. In addition, decreasing the gap (distance between the sensor shields) decreases the bit length, and therefore increases the amount of data that can be recorded on a given length of data track.
In this effort to decrease size and increase data capacity and data rate, data recording systems have been designed with ever smaller fly heights. The fly height is the distance between the air bearing surface (ABS) of the read/write head and the surface of the magnetic medium. This decreased fly height, however, greatly increases the chances of a head disk contact during operation. Such contact can be catastrophic for several reasons. For instance, such contact results in a heat spike in the head, which can lead to demagnetization of the pinned layer or loss of free layer biasing, rendering head unusable. In addition, data loss can occur due to damage to the magnetic medium during such contact.
The chances of such a head disk contact occurring at such low fly heights increase dramatically when a portion of the head protrudes from the ABS. Such protrusion can be the result of what is called “thermal protrusion”. Certain elements of the head, such as magnetic shields of the read head have thermal expansion coefficients that are larger than other portions of the head, such as the alumina insulation layers. As the ambient temperature or the temperature inside the disk drive increases, the shield, having a higher thermal expansion coefficient, will protrude out past the ABS. One possible solution might be to find a shield material that has a thermal expansion coefficient that is similar to the other materials making up the head. However, no materials are currently available that posses both the desired magnetic properties for a magnetic shield and also a desired thermal expansion coefficient.
Therefore, there is a strong felt need for a head structure that can reduce the thermally induced protrusion of magnetic shields in a magnetic head used in a data recording system. Such a system must not compromise magnetic performance including signal resolution such as the PW50 performance of the magnetic recording system.